lievent源码分析：evbuffer

struct evbuffer定义在evbuffer-internal.h文件中。

evbuffer结构内部保存一个以evbuffer-chain结构为节点的链表，evbuffer内部有两个分别指向首尾节点的指针。

 struct evbuffer {
     /** The first chain in this buffer's linked list of chains. */
     struct evbuffer_chain *first;
     /** The last chain in this buffer's linked list of chains. */
     struct evbuffer_chain *last;

     /** Pointer to the next pointer pointing at the 'last_with_data' chain.
      *
      * To unpack:
      *
      * The last_with_data chain is the last chain that has any data in it.
      * If all chains in the buffer are empty, it is the first chain.
      * If the buffer has no chains, it is NULL.
      *
      * The last_with_datap pointer points at _whatever 'next' pointer_
      * points at the last_with_datap chain.  If the last_with_data chain
      * is the first chain, or it is NULL, then the last_with_datap pointer
      * is &buf->first.
      */
     struct evbuffer_chain **last_with_datap;

     /** Total amount of bytes stored in all chains.*/
     size_t total_len;

     /** Number of bytes we have added to the buffer since we last tried to
      * invoke callbacks. */
     size_t n_add_for_cb;
     /** Number of bytes we have removed from the buffer since we last
      * tried to invoke callbacks. */
     size_t n_del_for_cb;

 #ifndef EVENT__DISABLE_THREAD_SUPPORT
     /** A lock used to mediate access to this buffer. */
     void *lock;
 #endif
     /** True iff we should free the lock field when we free this
      * evbuffer. */
     unsigned own_lock : ;
     /** True iff we should not allow changes to the front of the buffer
      * (drains or prepends). */
     unsigned freeze_start : ;
     /** True iff we should not allow changes to the end of the buffer
      * (appends) */
     unsigned freeze_end : ;
     /** True iff this evbuffer's callbacks are not invoked immediately
      * upon a change in the buffer, but instead are deferred to be invoked
      * from the event_base's loop.    Useful for preventing enormous stack
      * overflows when we have mutually recursive callbacks, and for
      * serializing callbacks in a single thread. */
     unsigned deferred_cbs : ;
 #ifdef _WIN32
     /** True iff this buffer is set up for overlapped IO. */
     unsigned is_overlapped : ;
 #endif
     /** Zero or more EVBUFFER_FLAG_* bits */
     ev_uint32_t flags;

     /** Used to implement deferred callbacks. */
     struct event_base *cb_queue;

     /** A reference count on this evbuffer.     When the reference count
      * reaches 0, the buffer is destroyed.    Manipulated with
      * evbuffer_incref and evbuffer_decref_and_unlock and
      * evbuffer_free. */
     int refcnt;

     /** A struct event_callback handle to make all of this buffer's callbacks
      * invoked from the event loop. */
     struct event_callback deferred;

     /** A doubly-linked-list of callback functions */
     LIST_HEAD(evbuffer_cb_queue, evbuffer_cb_entry) callbacks;

     /** The parent bufferevent object this evbuffer belongs to.
      * NULL if the evbuffer stands alone. */
     struct bufferevent *parent;
 };

struct evbuffer_chain：

evbuffer-chain结构内部保存一个表示buffer内容长度的变量以及一个char*的指针指向buffer内容所在的位置。

 /** A single item in an evbuffer. */
 struct evbuffer_chain {
     /** points to next buffer in the chain */
     struct evbuffer_chain *next;

     /** total allocation available in the buffer field. */
     size_t buffer_len;

     /** unused space at the beginning of buffer or an offset into a
      * file for sendfile buffers. */
     ev_misalign_t misalign;

     /** Offset into buffer + misalign at which to start writing.
      * In other words, the total number of bytes actually stored
      * in buffer. */
     size_t off;

     /** Set if special handling is required for this chain */
     unsigned flags;
 #define EVBUFFER_FILESEGMENT    0x0001  /**< A chain used for a file segment */
 #define EVBUFFER_SENDFILE    0x0002    /**< a chain used with sendfile */
 #define EVBUFFER_REFERENCE    0x0004    /**< a chain with a mem reference */
 #define EVBUFFER_IMMUTABLE    0x0008    /**< read-only chain */
     /** a chain that mustn't be reallocated or freed, or have its contents
      * memmoved, until the chain is un-pinned. */
 #define EVBUFFER_MEM_PINNED_R    0x0010
 #define EVBUFFER_MEM_PINNED_W    0x0020
 #define EVBUFFER_MEM_PINNED_ANY (EVBUFFER_MEM_PINNED_R|EVBUFFER_MEM_PINNED_W)
     /** a chain that should be freed, but can't be freed until it is
      * un-pinned. */
 #define EVBUFFER_DANGLING    0x0040
     /** a chain that is a referenced copy of another chain */
 #define EVBUFFER_MULTICAST    0x0080

     /** number of references to this chain */
     int refcnt;

     /** Usually points to the read-write memory belonging to this
      * buffer allocated as part of the evbuffer_chain allocation.
      * For mmap, this can be a read-only buffer and
      * EVBUFFER_IMMUTABLE will be set in flags.  For sendfile, it
      * may point to NULL.
      */
     unsigned char *buffer;
 };

 /** callback for a reference chain; lets us know what to do with it when
  * we're done with it. Lives at the end of an evbuffer_chain with the
  * EVBUFFER_REFERENCE flag set */
 struct evbuffer_chain_reference {
     evbuffer_ref_cleanup_cb cleanupfn;
     void *extra;
 };